Still shakin'

I am chasing the same thing. Mine starts at about 70 mph, gets heavy at 85 or so, horrendous at 100 and is practically undriveable at 110. I had it with my 318 and don't remember if it was a problem with the stock rear end. I had a terrible vibration the first time I drove it and knew it was the converter. I changed the converter and installed a Dana 60 at the same time. It was much better, but it came at speed now. I changed wheels and tires for fitment, and that didn't change it. I was driving the car to the shop to install the 408 and the fan broke two blades off. I thought "aha! The damn fan was unbalanced from the cracking!" I put in the 408 and 200R4 tranny. Vibration slightly better but still there. I installed an aluminum radiator and electric fans, still there. As soon as the snow melts I am going to check my pinion angle as well, but I know it was good on install. Worth checking again. I am leaning towards a slightly bent axle. I jack it up and let the wheels spin and I can see a slight bounce on the driver's side. I will keep in touch with what I find fixes mine, and I am subscribed to this post.